Pipe coupling

ABSTRACT

A pipe coupling or connection comprises a female coupling component and a mating male coupling component. Each component is matingly threaded for coupling engagement, and each is provided with an adjacent frusto-conical sealing surface. The sealing surface is provided with a controlled surface roughness, viz. very shallow fine surface variations, preferably formed as microgrooves at a pitch small relative to the pitch of the threads. The slope of the frusto-conical surface of the sealing area of the male component is slightly mismatched with that of the sealing area of the female component to simulate the bearing force vs. axial distance characteristic of shrunk-fit circular cylindrical sealing surfaces. The slight mismatch enables the sealing pressure to be above a design minimum throughout the entire sealing area yet higher at each end of the sealing area. As the coupling is assembled, thread interference in the vicinity of the sealing surface lags the occurrence of interference between the sealing surfaces. Load threads are provided with negatively inclined load flank faces.

RELATED APPLICATION

This application is a continuation-in-part of application Ser. No.07/467620 filed on Jan. 19, 1990 and now abandoned.

This invention relates to couplings or connections for use ininterconnecting lengths of pipe casing or tubing made of steel or thelike. In the following description, both terms "coupling" and"connection" are used, usually interchangeably without preference, itbeing understood that the invention applies equally to threaded andcoupled connections and to pin and box members integral with the pipeand interconnecting lengths of pipe.

BACKGROUND, PRIOR ART

In some applications, steel tubing is subjected to severe stresses, andwhere a series of lengths of pipe or tubing have to be coupled together,the connection or coupling itself must be able to bear the appliedstresses. For example, for use as oil well casing, such tubing may beused in conjunction with steam injection into the well wheretemperatures of the order of 650 degrees Fahrenheit may be reached. Thissubjects the tubing to compressive and tensile axial loads approachingor even exceeding the actual yield strength of the material in the pipebody. Thus, any connection or coupling for joining together successivelengths of pipe must be able to withstand the axial loading withoutfailure and still be resistant to leakage from internal pressuresapproaching the actual yield strength of the pipe body. As the pipe isalternately heated and cooled, the axial loading on the pipe andcouplings may become alternately compressive and tensile, and throughoutthe coupling must maintain its seal with the pipe ends in resistinginternal pressure.

Such couplings (connections) comprise a male and mating female couplingcomponent. The male component is a suitably configured threaded portionat at least one end of the steel pipe, constituting the pin member ofthe coupling. A mating annular female component long enough in the axialdirection to receive the pin ends of two adjoining lengths of pipe isinternally configured and threaded at each end for mating engagementwith the pin member, thereby completing the coupling. The annular femaleelement is often referred to as the box element or box member of thecoupling. Equally, one end of the pipe could be upset and internallythreaded to constitute the female component of the connection.

Conventionally, the pin member of the coupling or connection is taperedinwardly from the proximal end of the threaded portion to the distal endto mate with a similarly tapered female threaded member of the coupling.The taper facilitates entry of the pin member into the box member.

In pipe couplings, a seal is typically maintained. The seal may beeffected between the mating threaded portions of the pin member and thebox member of the coupling, but this kind of seal is subject to readyleakage. In other couplings, some auxiliary sealing element (e.g. anannular elastomer) is provided. In yet other couplings of which thepresent invention is a species, the axial load-bearing threaded portionsof the coupling do not themselves necessarily provide a seal; the sealis a separate metal-to-metal seal provided adjacent the axialload-bearing threaded portions, in both the pin and the box members ofthe coupling, or male and female elements of the connection.

The threaded axial load-bearing portion of the coupling (or connection)should conform to certain known design principles. The total axialbearing surface provided by the full-depth load-bearing threads shouldbe at least equal to the cross-sectional area of the pipe material. Theangle of orientation of the stab flanks of the axial load-bearingthreads should differ from the angle of orientation of the load flanksby not less than about 15 degrees. The step height of the axial loadbearing threads should be greater than the thread radius so as to avoidgalling (metal abrasion). Tilt of the distal end of the pin that causesthe yield strength of the steel to be exceeded is generally to beavoided. Other general principles of thread design and coupling designwill be known to those skilled in such design work and should be appliedto the design of the connection of the present invention.

Couplings are known in which the angle of orientation of the load flankof the axial load-bearing threaded portion is negative.

Couplings are further known in which a metal-to-metal seal is provided.For best results, such surfaces should have a controlled degree ofroughness, as by shot peening, grit blasting, glass bead peening, orhelical microgroove threads having a pitch very small relative to thepitch of the load-bearing coupling threads.

Problems exist in known pipe couplings. Especially, the pin end of thepipe is often subjected to rough handling with consequent damage tocoupling and sealing surfaces. A pipe coupling should be able tofunction effectively if there has been slight damage to the pin. Thiscould be accomplished if the sealing force were designed to be abovesome predetermined minimum throughout the effective sealing area, and iswell above the minimum value at each end of the sealing area. Then, ifeither end (or some point in between) of the sealing area were slightlydamaged, the seal at one end of the sealing area at least would providean adequate seal. But many prior couplings having metal-to-metal sealingareas adjacent the threaded coupling areas typically provide a minimumor even less than minimum design sealing force at one end of thecoupling and a maximum (above design sealing force) at the other. So ifthat part of the sealing area in which the sealing force is designed tobe maximum is damaged, the effective overall effective sealing force maybe only the design minimum sealing force, or conceivably even less thanthe design minimum. This may be insufficient to prevent leakage underextreme operating conditions.

SUMMARY OF THE INVENTION

The pipe coupling or connection according to the invention overcomes theforegoing problem by providing a metal-to-metal seal in which thesealing force is well above the design minimum at each end of thesealing area and at or above the design minimum throughout the sealingarea.

A pipe coupling or connection according to the invention is of the typecomprising a female coupling component and a male coupling component,each matingly threaded for coupling engagement, and each provided with asealing area adjacent the coupling threads (load threads). The sealingarea of each coupling component is formed as a frusto-conical surface,at least one and preferably both surfaces being slightly imperfect. Suchsurface is imperfect in the sense that it has a controlled surfaceroughness. In other words, it is provided over its surface with veryshallow, closely spaced fine surface variations or irregularities, asare formed for example by shot or glass bead peening, etching, gritblasting, or by threading with microgrooves at a pitch very smallrelative to the pitch of the coupling threads. The degree of surfaceroughness chosen will depend upon the application. For example, to avoidgas leakage, the surface irregularities or degree of roughness should byvery fine. In drilling mud, a coarser surface finish may besatisfactory. As thus far described, the structure is conventional.

The coupling or connection according to the invention differs from knowncouplings in that it has the following combination of characteristics,or such of them as may pertain to the achievement of the designobjective at hand:

1. The seal taper angle is of a low gradient, and there is a slightmismatch between the pin seal taper and the box seal taper, the box sealtaper being slightly steeper than the pin seal taper, so as to obtain asealing force distributed over the sealing area that simulates that of ashrink-fit cylindrical seal.

2. As the coupling is made up (assembled), interference of the pin andbox sealing areas occurs at least about as soon as, and preferablybefore there is any interference between the load threads of the pin andbox members in the vicinity of the sealing areas, ("Interference" inthis context means interference tending to pry apart the frusto-conicalsealing surfaces of the pin and box--i.e. interference creating a radialforce between these sealing surfaces).

3. The load flanks of the load threads are negatively inclined.

4. The sealing areas are coated with a high-temperature graphiteparticle-containing lubricant with a relatively high content of solidgraphite particles, or similar such lubricant.

5. The coupling is provided with an "insurance" auxiliary seal operatingwhen the pipe is in compression. (It may also operate for tensile loadsup to some threshold value). The "compression" auxiliary seal is formedbetween a slightly negatively inclined annular seat formed by a torqueshoulder at the proximal end of the box located inwardly of the sealingarea, and the terminating distal radial surface of the pin.

6. The pin seal taper may desirably be at the same angle to the pin axisas the pin load thread taper. The box seal taper is thus preferablyformed to be slightly steeper than the pin seal taper. The box loadthread taper may be at the same angle to the box axis as the pin loadthread taper, or may be slightly shallower, as proposed in copendingapplication Ser. No. 07/770,566, filed concurrently herewith.

These various characteristics of the pipe coupling of the invention willnow be discussed in more detail.

1. Pipe couplings are previously known in which the box seal taper issteeper than the pin seal taper. It is known that such mismatchfacilitates initial contact and deflection ("tilting") of the pin noseas it engages the box sealing surface during assembly, and tends toreduce the overall seal area, concentrating the pressure loading of thesealing area over a relatively small portion of the available sealingsurface in the vicinity of the distal end of the pin, as disclosed forexample in Mott U.S. Pat. No. 4,736,967, granted Apr. 12, 1988.

Mott discloses a seal taper differential between box and pin, but doesnot discuss seal taper relative to thread taper. Nor does Mott discussany preferred relationship between such tapers and the grade of steelused.

Furthermore, Mott does not teach a pin/box sealing area mismatch thatprovides an axial distribution of sealing force over the sealing areasimulating that which would be obtained from shrink-fitting an internalcircular cylindrical box sealing surface about a mating externalcircular cylindrical pin sealing surface. Such axial distribution ischaracterized by a relatively uniform sealing force (at or exceeding thedesign minimum) over most of the axial length of the contacting sealingsurfaces, but having force peaks well in excess of the design minimum atboth ends of the axial extent of the contacting sealing surfaces. Theseforce peaks are desirable because they provide the greatest sealingforce in the areas that are most sensitive to disturbance. At the distalend of the pin, there is a risk of damage due to careless handling. Atthe proximal end of the pin, there is the risk of seal separation due toout-of-tolerance load thread interference in the vicinity of the sealingarea. So the availability of peak sealing forces at both ends of thesealing area tends to offset these risks.

The seal taper of both pin and box seals must be of relatively slight(shallow) gradient. This facilitates maintenance of a relatively longsealing area in the axial sense and ensures that the desired simulatedshrink-fit cylindrical sealing characteristic is obtained. Further, itensures that tilting of the distal end of the pin during assembly of thecoupling will not be unacceptably severe, and that there is an adequatethickness of material at the distal end of the pin. (Some tilting of thepin end is desirable, because it provides the force peak at the distalend of the pin sealing area and causes some burnishing of that part ofthe sealing area to occur.)

Further, because there will be relative movement of box and pin sealingsurfaces as a result of compression/tension thermal cycling of the pipe,it is important to maintain adequate contact between the sealingsurfaces throughout the cycling. This cannot be accomplished if thetaper is too steep.

The mismatch between the box and pin tapers must be sufficient tosimulate the desired cylindrical shrink-fit seal characteristic, butshould not appreciably exceed that degree of mismatch. Furthermore, themismatch should not substantially interfere with the design objective ofmaintaining an adequate thickness of material at the distal end of thepin and of avoiding undue tilting of the distal end of the pin duringassembly. This result can be achieved if the taper (in inches per inch)mismatch is kept less than the designed gauge-point nominal interference(in inches) between the box and pin sealing areas, at least for most oilwell pipe. Preferably the mismatch should be about 60% of nominalinterference, or at most about 70%. (For the sealing areas, the gaugepoint is preferably selected to be about midway between the beginningand end of the longer of the two sealing areas, the longer sealing areabeing that of the pin for manufacturing reasons, and at a nearby matingpoint of the shorter of the two sealing areas, viz, the female sealingarea). As the degree of mismatch declines in the direction of equalityof pin and box sealing surface tapers, the force peak at the distal endof the pin sealing area drops off relative to the force peak at theproximal end of the pin sealing area. This result may be tolerable forsome applications, but it is preferred to have the mismatch sufficientto create a distinct substantial force peak at the distal end of the pinsealing area, since this objective tends to offset the possiblereduction in sealing efficacy of that part of the seal that may becaused by slight damage to the distal end of the pin. Note thatinterference should increase if the grade of steel increasesappreciably.

It is within the scope of the invention that only one of the matingsealing surfaces be roughened in the manner described, the other surfacebeing smooth. That arrangement will still provide a good seal. However,it is preferable that both surfaces have a controlled surface roughness.

2. At the proximal end of the pin sealing surface, a force peak isdesired when the coupling is made up (assembled). This objective will bedefeated or impeded if there is too much load thread interference in thevicinity of the sealing area. We are here referring to the radialinterference of box with pin threads adjacent the seal. Desirably thebearing force when the coupling is fully made up will tend to beconcentrated at the proximal end of the sealing area (relative to thepin), rather than borne by the load threads in the vicinity of thesealing area.

Furthermore, it is desired that there be some burnishing of the sealingsurfaces and mashing or compaction of the solid particles in the sealinglubricant during assembly. This objective cannot be achieved ifinterference between box and pin load threads prevents pressure contactbetween box and pin sealing surfaces during make-up.

If the simulation of the cylindrical shrink-fit seal load forcecharacteristic is achieved, it follows that the load thread interferencein the vicinity of the sealing area when the coupling is fully made upis not sufficient to pry apart the sealing surfaces of box and pin. Butthis design criterion is not in and of itself sufficient to achieve thelatter of the two objectives mentioned above.

Accordingly the relative dimensions, configuration and angles of loadthreads and sealing surfaces should be chosen so that box/pin sealingsurface interference occurs during make-up at least as soon as, andpreferably before, load thread interference in the vicinity of thesealing area occurs. (Load thread interference in the vicinity of theproximal end of the pin load threading is less critical, since it willnot usually have any appreciable tendency to pry apart the sealingsurfaces of box and pin). The greater the lag of load threadinterference following seal surface interference during make-up, thehigher the concentration of sealing force at the proximal end of the pinsealing surface.

3. Although the provision of negative load flank angles on the loadthreads is a characteristic of some previously known couplings, thepurpose of such provision has been to eliminate the sliding of the pinrelative to the box so as to tend to disengage the box and pin when thepipe is under tension. In other words, the negative load flank anglesare there to prevent the coupling from bursting apart when tensilestress is applied to the coupling.

It has not however heretofore been specifically taught that thecombination of negative load flank angles with the other structuralfeatures mentioned above improves the seal, in that it resists anytendency of the sealing surfaces to be pried apart when the coupling isplaced under tension.

The load flank faces should be slightly negatively inclined relative tothe radial direction, an angle of the order of -5 degrees (dependingupon pipe diameter and thread depth) typically being suitable. The stabflank faces of the coupling threads are then formed at a positive angleto the radial, and, as mentioned earlier, the sum of the stab flankangle to the radial and the load flank angle to the radial should not beless than about 15 degrees. So the stab flank angle could be a minimumof about +20 degrees, assuming a load flank angle not exceeding -5degrees (in a negative sense).

4. As mentioned, the coupling seal designed in accordance with theprinciples of this invention tends to be optimum if a high-temperaturehigh-solids graphite particle-containing lubricant or equivalent isapplied to the sealing surfaces before make-up of the coupling.Lubricants employing solid metallic particles (e.g. copper) are not assatisfactory because it is difficult during the sliding of the box andpin sealing surfaces during make-up to compress or break up the metallicparticles. Graphite particles are much easier to mash, and when they do,they tend to fill the hollows in the sealing surfaces. Even if thepetroleum constituent of the lubricant is later lost as a result of hightemperatures, the graphite remains to fill the voids and hollows betweenpin and box sealing surfaces.

5. Desirably, a torque shoulder is formed at the proximal interiorportion of the box against which the mating distal end of the pinthrusts and seals when the coupling is fully made up. The face of thistorque shoulder is preferably given a slight negative angle to impedeany tendency of the distal end of the pin to deflect or deform in aninward radial sense, and to force it into preferred sealing engagementwith the box.

Compression of the pipe coupling forces the distal end of the pinagainst the box torque shoulder, and if the facing surfaces arereasonably smooth and well-mated, a seal is formed that augments theseal formed by the contacting frusto-conical sealing surfaces of the boxand pin.

6. The design of load thread tapers to provide a slight mismatch betweenbox and pin thread pitch lines is more fully discussed in theaforementioned copending patent application Ser. No.07/770,566. The pinthread taper should, to achieve the desired increase in bearing load atthe proximal pin sealing surface, be slightly steeper than the boxthread taper.

However, the coupling design according to the invention has significantmerit in comparison with previous designs even if there is no threadtaper mismatch. The pin thread taper and box thread taper can beidentical. In that case, manufacturing convenience dictates that the pinsealing surface taper should preferably be identical with the pin threadtaper, and the seal mismatch effected by designing the box sealingsurface taper to be steeper than the pin sealing surface taper.

The coupling threads (load threads) of the male and female componentsmay be formed along thread pitch lines whose surfaces of revolution formmating frusto-conical surfaces (i.e. the thread pitch lines are taperedfor easy coupling). The slope of the frusto-conical surface along whichthe full depth coupling threads (load threads) of the male (pin)coupling element are formed should, for manufacturing convenience,preferably be equal to the slope of the frusto-conical sealing surfaceof the male coupling element. The female (box) element must of course beinternally formed to mate with the male, subject to the slight mismatchof sealing surface slopes previously mentioned, and subject to thepossibility of a slight mismatch between the slopes of the box and pinload threads, as described in more detail in copending application Ser.No. 07/770,566, filed concurrently herewith.

Note that the coupling threads of the male and female (pin and box)members of the coupling or connection do not necessarily provide a seal;the interference can be designed to be relatively low. Clearance shouldbe provided between the crest of the pin threads and the root of the boxthreads, but some interference between the root of the pin threads andcrest of the box threads, in accordance with conventional practice., maybe designed.

Alternatively, interference may result from a slight mismatch of box andpin load threads, as discussed in the aforementioned copendingapplication Serial No. 07/770,566.

The foregoing characteristics of the connection/coupling of theinvention, afford a useful bearing load-versus-length relationshipsimulating that of a shrink-fit cylindrical seal over the length of thesealing portion that is characterized by relatively high bearing load ateither extremity of the sealing portion of the pin and box members, andsomewhat lower bearing load (but at least as high as design minimum)intermediate the two ends of the sealing portion, when the box memberhas been fully threaded onto the pin member. What is considered to be asufficient axial length of the sealing area will be dependent upon suchparameters as pipe diameter, grade of steel, wall thickness variation,etc. For most pipe suitable for use in oil well applications, a pinsealing area of the order of one inch in length and a box sealing areaof approximately half or more of the length of the pipe sealing areawill be found to be satisfactory.

The foregoing characteristics ensure that a very good seal is maintainedat both ends of the sealing portion if the coupling and sealing surfacesof the pin and box members are relatively undamaged. There is, asmentioned, a risk of some surface damage, particularly at the distal endof the pin member. If that damage prevents the distal end of the pinmember from engaging the mating (but slightly mismatched) sealingportion of the box member with a sufficiently high bearing (sealing)force, there will still be adequate bearing force at the proximal end ofthe sealing area of the pin portion when the pin engages the mating boxmember, and thus, there will be adequate bearing pressure at least atone end of the sealing area to form an adequate seal.

Note that a greatly mismatched taper of pin member relative to boxmember sealing surfaces would give a high bearing (sealing) load at thedistal end of the pin member but a relatively low bearing load at theproximal end of the pin member. This is undesirable, especially becausethe risk of damage is highest to the distal end of the pin. Alsorelatively undesirable is the case of identical tapers of pin and boxmembers, affording a high bearing (sealing) load at the proximal end ofthe pin member, the bearing load diminishing toward the distal end. Theinvention should be implemented between these two extremes.

For most oil well pipe, to ensure that the mismatch is within acceptablelimits, as mentioned above, the slope of the box member sealing areaover a unit length should exceed that of the pin by less than thenominal interference between the threaded sealing areas at or near thegauge point. This would be a preferred characteristic for pipes of 3.5inches to about 16 inches in diameter, and having contacting sealingareas of at least about 3/8 inch.

A further preferred characteristic of the connection or couplingaccording to the invention is that the relatively long and low-angledsealing surfaces have sufficient interference at the gauge point of eachmember to be greater than the pressure differential for which theconnection or coupling has been designed (which according to thepreferred embodiment, and in accordance with conventional designpractice, is 100% of yield strength of the pipe body) regardless of thebearing load at either end of the sealing threaded portion, when eitheraxial tension or axial compression loads are applied to the coupling.

The preferred low-angled sealing area design, of the pin elementespecially, is beneficial in that during handling, if there is anydamage, e.g. from stabbing the pin into the box, any unwantedprotrusions or other damaged parts of the distal end of the pin may behand dressed, i.e. filed off or ground off without any appreciablediminution of the efficacy of the coupling. The proximal end of the pinin engagement with the mating coupling threads of the box can beexpected to have a sufficiently high bearing load that such load willexceed the pressure differential for which the coupling has beendesigned, even at full pipe yield pressures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an axial partial section view of a pin member wall constructedin accordance with the invention, at the end of a length of pipe, asseen through the pipe wall, showing the coupling threaded portion andadjacent sealing portion for engaging a mating box member of thecoupling.

FIG. 2 is an axial partial section view of a wall portion of the matingfemale or box member of a coupling according to the invention showingthe coupling threaded portion and sealing threaded portion for receivingthe pin member of FIG. 1.

FIG. 3 is a partial view in axial section of a modified alternativeconstruction of the wall portion of the box member modified to permitchaser manufacture of the box threads, and otherwise conforming to thebox structure of FIG. 2.

FIG. 4 is a detailed enlarged partial axial section view of the pinthread of FIG. 1, taken at the gauge point.

FIG. 5 is a detailed enlarged partial axial section view of the boxthread taken at the gauge point of the box of FIG. 2.

FIG. 6 is a detailed enlarged partial axial section view of the couplingthread of FIG. 4 shown engaging the coupling thread of FIG. 5, underload, assuming equal taper of pin and box load thread (i.e. having thesame thread pitch line).

FIG. 7 is a greatly enlarged detailed partial axial section view of aportion of a microgroove sealing area suitable for use as the sealingarea of either the pin member or box member of FIG. 1.

FIGS. 8A through 12B are a series of graphs and schematic views of therelative degrees of taper of pin and box sealing areas of the pin andbox members of the coupling according to a preferred embodiment of theinvention (FIGS. 9A through 9D), such preferred embodiment beingpresented in FIGS. 1 to 6, when the pin member has fully engaged the boxmember, as compared with alternative configurations whosecharacteristics are shown in the remaining ones of these figure. Thedimensions, slopes and mutual spacing of the pin and box sealing areashave been exaggerated for ease of comprehension. The orientation ofFIGS. 8A through 12B is the same as that of FIGS. 1 and 2, with thedistal end of the pin to the right, and the proximal end of the pin tothe left.

FIG. 8A is a graph showing a desirable shrink-fit circular cylindricalseal bearing load vs. distance characteristic.

FIG. 8B schematically represents the box and pin sealing arearelationship giving rise to the characteristic depicted in FIG. 8A.

FIG. 9A is a graph showing a representative seal bearing load vs.distance characteristic for the seal of a coupling or connectiondesigned according to the principles of the invention, where threadinterference in the vicinity of the sealing area begins aboutsimultaneously with seal surface interference, as the coupling is beingmade up, and where box thread taper is the same as pin thread taper.

FIG. 9B schematically represents the slightly mismatched gently slopedtapers of box and pin sealing surfaces according to the invention,giving rise to the graphs of FIGS. 9A, 9C and 9D.

FIG. 9C is a graph showing a representative seal bearing load vs.distance characteristic for the seal of FIG. 9B, but where threadinterference in the vicinity of the sealing area lags the occurrence ofsealing area interference as the coupling is being made up.

FIG. 9D is a graph showing a representative seal bearing load vs.distance characteristic for the seal of FIG. 9B as modified in the sameway as for FIG. 9C, but with pin thread taper being slightly steeperthan box thread taper.

FIG. 10A is a graph showing a representative seal bearing load vs.distance characteristic for a coupling box seal/pin seal relationship inwhich both box and pin sealing surfaces have the same gently slopedtaper.

FIG. 10B schematically represents the box and pin sealing arearelationship giving rise to the characteristic depicted in FIG. 10A.

FIG. 11A is a graph showing a representative seal bearing load vs.distance characteristic for a coupling box seal/pin seal relationship inwhich both box and pin sealing surfaces have the same steeply slopedtaper.

FIG. 11B schematically represents the box and pin sealing arearelationship giving rise to the characteristic depicted in FIG. 11A.

FIG. 12A is a graph showing a representative seal bearing load vs.distance characteristic for a coupling box seal/pin seal relationship inwhich the box and pin seal are greatly mismatched.

FIG. 12B schematically represents the box and pin sealing arearelationship giving rise to the characteristic depicted in FIG. 12A.

DETAILED DESCRIPTION OF THE INVENTION

The end of a steel pipe, tube or casing 11 is formed to provide a pingenerally indicated as 12. Pin 12 has a threaded portion 19 beginning ata chamfered starting thread 15 located at a position short of the distalend 18 of the pipe 11 and extending axially therefrom to terminate in avanish point 13. The thread pitch line of threaded portion 19 of the pin12 is sloped inwardly from its proximal end at vanish point 13 towardits distal end. The pin 12 terminates in a frusto-conical sealing area21 provided with a controlled surface finish to provide a limited degreeof roughness, e.g. helical microgrooves formed by way of threading, asmore particularly illustrated in FIG. 7. The angle of slope of thesealing surface 21 along the frusto-conical surface is equal to that ofthe thread pitch line of the threaded portion 19.

It will be noted that in FIG. 1, the depth of the roots, and the heightof the crests of the threads of threaded portion 19 relative to theroots of the threads, of pin 12 increases from the vanish point 13 to amaximum about midway along the axial length of coupling portion 19, wellbefore reaching the starting thread 15. As illustrated, seven of thethreads are perfect threads.

It can be seen from referring to FIG. 4 that the load flank face 25 ofthe pin threads is negatively inclined at a very slight angle, shown asapproximately -3 degrees, to the radial plane 27. This load face angleorientation is maintained throughout the entire threaded portion 19 ofpin 12. The angle of inclination of the load flank face chosen will varywith choice of thread height, but can generally be expected not toexceed (in a negative sense) -10 degrees even for very shallow threads.

It can also be seen from FIG. 4 that the stab flank face 29 of the pinthreads has a positive angular orientation relative to the radial plane27. According to preferred design practice, the difference in anglebetween the orientation of the load flank face 25 and the stab flankface 29 should not be less than about 15 degrees. So if, for example,the load flank face 25 is at a negative angle of -3 degrees, then theangle of orientation of stab flank face 29 should not be less than about18 degrees. The stab flank face angle or orientation is also maintaineduniformly throughout the length of the threaded portion 19 of the pinmember 12.

Referring to FIG. 2, the box 31 of which half of a complete wall length(in the axial direction) is illustrated in FIG. 2, is internallyconfigured and threaded to mate with the pin 12 of FIG. 1. The otherhalf of box 31 (not illustrated) is similarly internally configured andthreaded to receive the pin of the next length of pipe. In an integralconnection, the pin could be formed as illustrated in FIG. 1, the femaleend as illustrated in FIG. 2 (or FIG. 3, as an alternative to FIG. 2).

Specifically, the female coupling element 31 is provided beginning atits distal end 33 with a threaded portion generally indicated as 35extending into the interior of box member 31 as far as a terminatingthread 38. Further inwardly from thread 38 is a gap functioning as asingle-point threading tool relief groove, generally indicated as 39,terminating in a shoulder 41 which defines the outermost limit of aninterior frusto-conically shaped , microgroove sealing surface generallyindicated as 43, which terminates in a limit or torque shoulder 45forming a negatively inclined annular seat 46. The negative inclinationof seat 46 tends to prevent the pin end 18 from climbing over theshoulder 45 when excess torque or high axial loading is applied to thecoupling.

Although the threaded portion 35 of box 31 and the sealing surface 43 ofbox member 31 are both tapered so as to receive in coupling and sealingengagement the mating pin 12 of FIG. 1, nevertheless the degree of taperof the interior sealing surface 43 of box 31 is deliberately chosen tobe slightly steeper than the degree of taper of the mating sealingsurface 21 of pin 12. The reason for this is to provide a preferredbearing load-versus-length relationship, as discussed above and to bediscussed in greater detail below with reference to FIGS. 9A and 9B.

The threads 35 (load threads) of box 31, shown in enlarged profile inFIG. 5, are angled to mate exactly with the threads of pin 12. Further,the thread pitch line of threads 35 is at least approximately that ofthreads 19. In other words, the surfaces of revolution of the threadpitch lines for the coupling threads of the male and female couplingcomponents are mating or nearly mating frusto-conical surfaces. (Theymay be slightly mismatched, as described in the aforementioned copendingapplication Ser. No.07/770,566). The box thread is deeper than the pinthread so as to afford the necessary thread clearance. Their respectivedimensions are better understood by referring to FIG. 6.

It can be seen from FIG. 6 that a representative load flank face 25 of arepresentative pin thread 26 is in flush engagement with a mating loadflank face 51 of a thread 52 of box 31. Between the crest 28 of thread26 and root 53 of the mating box member threaded portion 35 is aclearance gap 61. Further, there is a clearance gap 63 between stabflank face 29 of thread 26 and mating stab flank face 55 of matingthreaded portion 35 of box 31. On either side of thread 26, it can beseen that the crests 57 of the box threads 35 are in flush engagementwith the root faces 24 of pin coupling threads 19.

In other words, there is interference and flush engagement between thecrests of the box thread and roots of the pin thread, and compressiveinterference between the load flank faces of box and pin threads (whenthe threads are fully engaged and normal tensile stress occurs in thecoupling). In the same condition, there is clearance between the pinthread crests and box thread roots and clearance between the stab flanksof the box and pin threads. Note, however, that if the coupling were notstressed in tension but instead were stressed in compression, the stabflank faces rather than the load flank faces of the pin and box threadswould become in load-bearing contact with one another.

FIG. 3 illustrates an alternative structure for the interior of the boxmember. In FIG. 2, the coupling threaded portion was shown to terminatein a final thread crest 38, followed by a thread relief groove 39,followed by a curved shoulder 41, and sealing surface 43. By contrast,the relief groove 39 is omitted in the FIG. 3 alternative embodiment,and instead, there is a thread run-off area 73 intermediate the end ofthe coupling threaded portion 35 and the sealing surface 44, mergingwith surface 44 via a curved shoulder transition portion 75. The sealingsurface 44 of FIG. 3 is substantially identical to the sealing surface43 of FIG. 2, with the qualification that the total axial distanceoccupied by the sealing surface 44 of FIG. 3 is somewhat shorter thanthe total axial distance occupied by the sealing surface 43 of FIG. 2.

While the FIG. 3 embodiment has less total sealing area than the FIG. 2embodiment, nevertheless the FIG. 3 embodiment is easier to manufactureusing a chaser technique, using the same tool bit (requiring nowithdrawal of one tool bit and insertion of a separate tool bit). Thesealing surface 44 can, using the chaser technique, be machined first asa helical microgroove surface, immediately followed by the machining ofthe threaded portion 35, without withdrawing the tool. The FIG. 2embodiment does not admit of this possibility, but would require threeseparate tool bits to cut the threads, the thread relief groove, and thesealing area respectively, assuming that microgrooves are formed on thesealing area.

In this specification, reference will occasionally be made to the gaugepoint of the load threads and of the sealing surfaces of both pin andbox. This is the point at which nominal design values are selected forwhatever parameters pertain to such point. For example, the nominalinterference value designed for the coupling is selected relative to thegauge points of the box and pin--the sealing surface gauge points forseal interference, the load thread gauge points for thread interference.

The selection of the gauge point is arbitrary to some extent, butordinarily conveniently chosen as some intermediate point rather than aterminating point (of sealing surface, or threading, as the case maybe). Suitably selected gauge points are shown schematically in FIGS. 1and 2. Pin thread gauge point 20 is selected to be at or near themid-point of the range of perfect threads on the pin. Box gauge point 36is selected to be approximately aligned with pin gauge point 20 when thecoupling is made up. Pin seal gauge point 16 is selected to be in thevicinity of the mid-point (axially) of the effective pin sealingsurface. Box seal gauge point 42 is selected to be approximately alignedwith pin seal gauge point 16 when the coupling is made up.

In use, the pin 12 of FIG. 1 is stabbed into the opening generallyindicated as 47 of the box 31. Pin 12 is thrust in sufficiently far thatcontact is made between the starting thread 15 and a contacting threadsurface of the threaded portions 35 of box 31, following whichengagement of the threaded portions 19, 35 of pin 12 and box 31respectively begins. The box 31 is then rotated relative to pin member12 or vice versa so as to screw the pin member 12 into the box member31. Rotation of the box member 31 relative to pin member 12 continuesuntil the limit of the threaded portions is reached and the couplingthreaded portion 19 of pin member 12 fully engages the mating couplingthreaded portion 35 of box member 31. Rotation is effectively terminatedwhen distal end 18 of pin 12 comes into pressure contact with annularseat 46 of box 31. This contact, assuming that the distal end 18 of pin12 matingly seats against torque shoulder 45 in annular seat 46, willtend to form an effective auxiliary seal when the coupling is undercompression or in tension up to some threshold tensile load.

Before this point is reached, the sealing portion 21 of pin 12 will havecommenced engagement with the mating (but slightly differently tapered,as mentioned above and discussed in further detail below) interiorsealing surface 43 (or 44) of box 31. If the sealing areas aresurface-roughened by microgroove machining, it is apparent that thepitch of the microgrooves on sealing surfaces 21, 43 of pin 12 and box31 respectively must be very much smaller than the pitch of the threadedcoupling portions 19, 35. It follows that the microgrooves on pin 12will skip relative to the microgrooves of box 31, as the box 31 isscrewed onto pin 12. This action generally will not damage the sealingsurfaces 21, 43 appreciably, but will tend to smooth out any surfaceirregularities and will also, if a sealing compound has been applied tothe sealing surfaces, tend to spread the sealing compound over thesealing surfaces and cause entrapment of the sealing compound bydepressions in the mating sealing surfaces 21, 43 of the pin 12 and box31 respectively so as to facilitate formation of a large effectivesealing area as between the microgroove sealing surface 21 andmicrogroove sealing surface 43. The entrapment of sealing lubricant willserve to protect against wear and will reduce any propensity of thesealing areas to gall destructively.

As mentioned previously, a high-temperature high-solids graphiteparticle containing sealing lubricant is preferably used. As thecoupling is made up, the relative sliding action under pressure of thepin and box sealing surfaces tends to mash the graphite particles andforce them to occupy the hollows and voids in and between the engagingsealing surfaces, promoting effective sealing.

According to the invention, the thread configuration, sealing surfaceconfiguration and designed interference are selected so that when thepin sealing surface 21 first makes interfering engagement with the boxsealing surface 43, as the coupling is being made up, there has not yetbeen any undue interference between pin threads 19 and box threads 35 inthe vicinity of the sealing surfaces 21, 43. (Such undue threadinterference would undesirably prevent the desirable relative slidingaction under pressure between the sealing surfaces 21, 43).

There need never be any interference between the pin threads 19 and boxthreads 35 in the vicinity of the sealing surfaces 21, 43, or at all, sofar as creating an effective seal is concerned.

The profile of a microgroove surface suitable for use as the controlledroughened surfaces of sealing areas 21, 43 of pin member 12 and boxmember 31 respectively is shown (in profile) in the partial section viewof FIG. 7. The pattern is an undulate (wave) pattern with sharp narrowcrests and relatively wide shallow concavely curved troughs. The pitchof the helical microgrooves 21, 43 on pin member 12 and box member 31respectively is very small relative to the pitch of the couplingthreaded portions 19, 35 of pin member 12 and box member 31. Forexample, it may be of the order of 0.01 inches per revolution, ascompared with a pitch of 0.200 inches per revolution for the loadthreads, in the case of about 5 to 10-inch diameter pipe. The depth ofthe microgrooves (the distance between the peak of the crest and theroot of the trough) will vary with the pitch and type of cutting toolchosen. The depth of surface irregularities may be expected to beanywhere from about 30 to 250 microns, depending upon application.

It is not essential that the surfaces of sealing areas be formed ashelical microgrooves. Any similar undulate, mottled or roughened surfacewould suffice. The surface may be formed by any suitable technique suchas acid etching, ball or glass peening, or grit blasting. What isrequired is a fine, shallow series of surface variations orirregularities that create very shallow hills and valleys, relativelyfinely formed on the frusto-conical surfaces. This kind of formationtraps sealing compound and typically results in the prevention ofgalling of metal, as "hill summits" or crests yield as the pin iscoupled to the box, thereby improving the seal by reducing the depth ofthe microgrooves or other depressions. The controlled surface rougheningcharacteristics may depend upon application and environment. Gas-tightseals require a shallower depth of surface depressions than liquid-tightseals. It is obvious that the surface roughening should not create anaxially extending trough (through which leakage could occur) from oneend of the sealing area to the other.

FIGS. 8A through 12B illustrate graphically the effect of mismatchingthe tapers of the sealing portions of the pin and box respectively, bycomparing alternative selections of relative box and pin taper. Thefigures with suffix letter B depict schematically in axial section, andwith exaggeration of slope angles and dimensions, the opposing effectivesealing areas of the pin and box, showing the complete axial length ofmating sealing (e.g. microgroove) portions. Vertically aligned with eachof these schematic depictions is at least one graph, the abscissa ofwhich is the distance along the sealing area of the pin and box whilethe ordinate is the bearing load on the sealing surfaces when the pinhas been fully screwed into the box. (At that point, the pin distal face18 makes contact with annular seat 46 of the box 31.) The bearing loadconstitutes the sealing force, and is a measure of the efficacy of theseal.

If the tapers of the box sealing area and the pin sealing area arechosen to be identical, and zero, so that circular cylindrical sealingsurfaces are presented, then the plot of bearing load against axialdistance is that appearing in curve A of FIG. 8A. The interference as aconsequence of the shrink-fit of box on pin is designed to be sufficientto provide a minimum bearing load M throughout the axial length of themating sealing areas. The minimum value M is appreciably exceeded at thetwo ends. The FIG. 8A design is impractical, because engagement can beeffected only by shrink fitting. But the characteristic curve A is amodel to be emulated because of the load peaks at both ends of thesealing area and the uniform loading therebetween.

According to the invention, the slopes of the tapered pin and boxsealing surfaces are selected to be gently angled and slightlymismatched, as appears schematically in FIG. 9B. In that case, thebearing load varies with axial distance over the sealing area accordingto curve B1 in the graph of FIG. 9A. The bearing load is at a designminimum M at an intermediate point along the sealing threaded portionsof the pin and box and rises to a significantly higher value at bothends of the sealing threaded portion of the pin and box. This curve B1is the best available simulation of curve A of FIG. 8A obtainable fromany of the designs of FIGS. 9B, 10B, 11B, 12B.

It has been assumed in depicting the graph and physical arrangement ofbox and pin of FIGS. 9A, 9B that there is some thread interference inthe vicinity of the sealing surfaces when the coupling is made up. Ifthread interference is deliberately designed to lag the occurrence ofsealing surface interference by a considerable distance as the couplingis being made up, a superior result is obtained, viz. that of FIG. 9C.In that case, curve B2 is essentially similar to curve B1 over most ofthe axial distance along the sealing area, but curve B2 rises to asignificantly higher bearing force value than does curve B1 in thevicinity of the proximal end of the sealing area relative to the pin.This extra measure of proximal-end sealing force tends to cause someburnishing of the sealing surfaces where that force is present andfacilitates mashing of sealing compound also.

It is not necessary, even when the coupling is completely made up, thatthere be any thread interference in the vicinity of the sealing area. Insuch case, proximal pin sealing surface bearing load condiderablyexceeds the bearing load that would result if adjacent threadinterference were to lag sealing surface interference by only a slightdelay during make-up ("delay" of course being used in a relative box/pinrotational movement sense, not in an absolute time sense).

If there is a slight mismatch between pin load thread and box threadtaper, the pin thread taper being slightly steeper (more inclined to theaxial) than the box thread taper, as taught in the aforementionedcopending patent application Ser. No. 07/770,566, such that the threadtaper mismatch facilitates the avoidance of thread interference in thevicinity of the sealing area, then the result is a further increase inthe value of the bearing force (sealing force) at the proximal end ofthe sealing area relative to the pin. The result is graphically depictedin FIG. 9D.

If the tapers of the pin and box sealing surfaces are chosen to be equalbut gently angled (FIG. 10B) then the bearing load against distance plotalong the sealing area would appear as curve C in the graph of FIG. 10A.Bearing load would be highest at the proximal end of the effective pinsealing surface and lowest (not much higher than design minimum M) atthe distal end of the pin sealing surface. While the design of FIG. 10Bhas some value, it affords uncomfortably little margin at the distal pinend to accommodate variations in tilt of the pin sealing area thatresult from manufacturing tolerances. There is also a risk that the pindistal end bearing load may fall below design value M.

The term "tilt" used in the jargon of pipe connections refers to thechange in pin seal taper occurring at the distal end of the pin uponassembly of the pin into the box. Because of the taper of the pin, thepin wall tends to become thin at the distal end, and of course the steelis elastic, so the distal portion of the pin wall deforms (strains) asthe pin is threaded into tight engagement with the box. The degree oftilt will depend upon pipe diameter, interference, starting taper, sealarea length, etc. If, because of manufacturing tolerances of the designof the tapers selected, the tilt of the pin is greater than expected,the sealing force (bearing force) at the distal end of the pin may fallbelow design minimum value M if the FIG. 10B configuration is selected,but this is not likely if the FIG. 9B configuration is selected.

Some tilt of the distal end of the pin is desirable during make-up ofthe coupling, since that tends to facilitate mashing of the sealinglubricant solid particles and burnishing of the sealing surfaces. Buttoo great a tilt, which can be caused by too great a mismatch betweenbox and pin sealing surfaces, can cause undesirable strain of the distalend of the pin beyond the yield strength of the steel, in turn causingpermanent deformation and possible loss of distal end bearing load.

Note that because both box and pin seal surface tapers are relativelyshallow, the interference between box and pin sealing surfaces does notunacceptably increase as the coupling is made up. As the pin advancesinto the box, although the force between the pin and the box due tointerference increases, that force is borne by an axially longer sealingarea as the pin advances. Consequently, the point-to-point forces do notbecome unacceptably large. Eventually as the pin advances towards itsfinal point of contact against the torque shoulder of the box, there issome tilting of the distal end of the pin, which of course relieves thestress on the tilted portion.

The choices of interference, degree of tilt, and box and pin tapers,will vary according to the designer's preference, and due regard will bepaid to the grade of steel employed. As the grade increases, thedesigner may wish to increase the angle of the box taper relative tothat of the pin. This will tend to ensure that the distal end of the pinmaintains a tight contact with the torque shoulder of the box and willnot disengage as the coupling if finally made up. Higher grades of steelhave the capacity to absorb more energy from increased interference thanlower grades, and thus can tilt further than would be the case for lowergrades of steel. Typically the box taper chosen will be from about 3° to4° for grades of steel of yield strengths from 40,000 to 80,000 psi, andthe taper angle will be from 4° to 5° for grades of steel having yieldstrengths from about 80,000 to 150,000 psi. The 150,000 psi materialwill require more interference than say 50,000 psi material. Theincreased interference prevents the distal end of the pin from movingout of engagement with the box. It is important that internal pressureof gas (say) within the pipe coupling should not penetrate between thesealing surfaces to attempt to pry them apart. The coupling designaccording to the present invention is relatively free from such riskbecause of the significant amount of tilting of the distal end of thepin that occurs upon make up.

FIGS. 11A and lib reveal that the design of FIG. 10B can become totallyunacceptable if the degree of taper of the mating sealing surfaces issufficiently large. Where the sealing surfaces are steeply inclined, thebearing load diminishes to zero well before the distal end of the pinsealing surface is reached, and is above design minimum M only over anunacceptably short sealing area. The falling of curve D below theabscissa indicates absence of surface contact between the box and pinsealing areas. (In fact, the bearing load cannot fall below zero, so theportion of the curve below the abscissa is imaginary).

If the tapers of pin and box sealing surfaces are greatly mismatched,the sealing portions of the pin and box tend to lose surface contact andbearing force at the proximal end of the sealing portion of the pin,corresponding to the outermost end of the sealing area of the box. Thehighest bearing force in the case of too much mismatch is to be found atthe distal end of the pin, where adequate contact between the pin memberand box member threaded portions can occur. This is manifest in thegraph of FIG. 12A, in which curve E1 reflects the increase in bearingload from its minimum near the proximal end of the pin sealing surface(undesirably, below design load M) to a maximum at the distal pin end,relative to the greatly mismatched taper design of FIG. 12B. Curve E2illustrates a mismatch sufficiently severe that contact between the pinand box sealing areas is lost at the proximal end of the pin. (Again,the portion of the curve below the abscissa is imaginary).

By contrast to FIG. 12B, the tapers of both pin and box sealing areasare relatively gentle in the design of FIG. 9B, albeit slightlymismatched. If however the sealing surfaces were sharply sloped, as inFIG. 11B, the bearing load at the distal end of the pin sealing surfacewould tend to fall below acceptable design minimum value M. So theinvention should be practised under the condition that design minimumbearing (sealing) force M is exceeded throughout the axial length of themating sealing surfaces. Beyond one extreme of the preferred range,force M would be just slightly exceeded at the distal end of the pin.That would offer little or no improvement over the FIG. 10Bconfiguration. Just beyond the other extreme of the range, as oneapproaches the FIG. 8B design, it becomes impossible to fit the pin intothe box without severe galling. The invention is optimally practisedabout half-way between these extremes.

In all the graphs of FIGS. 8A through 12A, the premise has been assumedthat at any point along the effective sealing area, the bearing loadshould always be equal to or above some predetermined minimum acceptabledesign bearing load M, if at all possible, so that no matter where onelooks at bearing load, one will find that predetermined minimum equalledor exceeded. This minimum is equalled or exceeded at all points alongeach of the curves A, B1, B2, B3 and C. The criterion cannot be obtainedin the too sharply sloped configuration of FIG. 11B, and it may not beobtained in the FIG. 12B configuration. The minimum value M is desirablyappreciably exceeded at both ends of the sealing area, as illustrated incurves A, B1, B2 and B3, since leaks tend to develop at the ends. Theconfiguration is which this desideratum is optimally obtained is theslightly mismatched taper configuration of FIG. 9B, with furtherimprovement available if thread interference lags seal interferenceduring make-up, and if slight thread taper mismatch is selected aspreviously discussed.

In all of FIGS. 9B through 12B, it is assumed that the degree of taperon the internal sealing surface of the box is equal to or exceeds thedegree of taper on the sealing portion of the pin. A mismatch in theother sense, in which the taper in the box is less pronounced than thetaper on the pin, would be quite unsatisfactory, because then the pinmember would have difficulty adequately penetrating the box member, andundesirably high interference would result.

Note that if the distal end of the pin member is damaged in handling,such that an insufficient bearing load is present at that end, therewill nevertheless be an adequately high proximal pin end bearing load,if the slightly mismatched taper arrangement according to the inventionis selected. The invention, however, gives the additional advantage thatif the distal end of the threaded portion of the pin is only slightlydamaged, then hand dressed, an adequately tight seal is created at thedistal end as well as the proximal end, whilst in between, the sealingforce is everywhere higher than the minimum bearing force for which theseal has been designed.

The FIG. 9B selection of box and pin sealing area tapers tends to beoptimally resistant to surge stresses, and tends to afford optimalopportunity for hand-dressing repair of damaged sealing area surfaces.

As mentioned, with reference to a sealing area gauge point chosenintermediate the ends of the contacting sealing areas, the degree ofmismatch should be kept less than the gauge-point nominal interferencebetween the box and pin sealing areas, in order to obtain a curve B1, B2or B3 bearing load vs. distance characteristic rather than a curve E1characteristic. The nominal interference will be related to the yieldstrength of the steel. A mismatch of the order of 50 or 60% of nominalinterference is likely to be optimal.

Due regard must be paid to permitted tolerances in the chosen design ofcoupling according to the invention. Tolerances should be chosen forboth sealing surfaces and load threads that tend to minimize risk ofgalling of the sealing surfaces during make-up of the coupling. On theother hand, tolerances should not be chosen that would make possible areduction of bearing load throughout the effective sealing area belowdesign minimum. The design minimum normally should be at least equal tothe expected pressure differential at the yield strength of the selectedsteel. More tolerance is permitted for higher grade steel than for lowergrade steel.

EXAMPLE 1

Pin and box members according to the foregoing description were preparedfor use in couplings for 7-inch pipe having wall thickness ratings of 23and 26 pounds per foot. Such a coupling is intended for use with wellcasings where steam injection within the casing is required. Dependingupon the length of pipe and the expected pressures, a 55,000 psi minimumyield strength or 80,000 psi minimum yield strength steel may beselected. Temperatures up to 650 degrees Fahrenheit must be withstood,and axial tensile and compressive loads are expected to occur whichapproach or even exceed the actual yield strength of the material in thepipe body. The coupling was designed to withstand this axial loadingwithout failure whilst maintaining adequate resistance to leakage frominternal pressures ranging up to actual yield strength of the pipe wall.

The coupling was prepared with approximately twelve complete turns ofthreads tapered at 0.104 inches per inch for the pin and box, and havinga pitch of 0.200 inches per revolution. Of the twelve threads on thepin, seven were perfect threads, and the other five were partial threadsdiminishing to the vanish point 13 as illustrated in FIG. 1. The flankface orientation for the pin threading was the same as that for the boxthreading, namely -3 degrees for the load flank and +18 degrees for thestab flank.

For the sealing surfaces, the microgrooves were formed by a 3/64 inchradius turning tool fed at an axial feed rate selected within the rangeof about 0.008 inches to 0.015 inches per revolution. If the couplingwill be used in a gaseous environment, such as a heavy oil steamenvironment, a feed rate nearer the lower value (0.008inches/revolution) is preferred. For leak resistance in a conventionaloil environment, a feed rate nearer the higher value (0.015inches/revolution) is preferred. The total sealing length of the pinmember was selected to be 0.900 inches; the sealing portion in the boxmember would be slightly smaller, depending upon whether the FIG. 2 orFIG. 3 embodiment is chosen. The microgrooves at the 0.008inches/revolution pitch are about 0.0002 inch in depth and at 0.015inches/revolution pitch are about 0.0006 inch deep. The crests tend tobe flattened upon tightening the coupling, perhaps removing at leastabout 20% from the trough depth, and more in the vicinity of the area ofhighest bearing pressure.

The gauge points for the pin and box were selected as follows:

The pin and box thread gauge position was selected to be at a pointaxially where the threads in the made-up (assembled) position of pin andbox members were directly coincident and spaced from the last full-depththread (after which only partial flank depth occurs, diminishing towardsthe vanish point). This axial position was also selected so as to affordgreater than zero interference at the ends of the threaded portions inthe vicinity of the outer end of the box, less as one progressesinwardly (because of the load thread taper mismatch between box andpin).

The pin seal gauge point 16 was arbitrarily chosen to be 3/8 (0.375)inch from the distal end of the pin. The box seal gauge point 42 wasalso arbitrarily selected to be 0.375 inch from the box seat 46 (seeFIG. 2). The calculated minimum interference load M was then based uponthe nominal interference (0.019 inch) at a location 0.375 inch from thedistal end of the pin which coincides exactly with gauge point of thebox seal. Selecting the gauge point is somewhat arbitrary on the basisthat to stop leakage, the interference at any point along the activeseal must be equal to or greater than the minimum seal interference(0.012" in this case). Since the coupling has unequal seal tapers forthe pin and the box, in this case it is more convenient to locate thegauge points at the same axial location from the torque shoulder.

The pin sealing area taper was 0.104 inches per inch on diameter, thesame as that of the thread pitch line of the threads, whilst the boxsealing area taper was 0.1146 inches per inch on diameter. This is amismatch of 0.0106 inches per inch on diameter, or less than the minimumgauge point sealing area interference of 0.012 inches on diameter. Thetolerance of the pin thread and seal at the gauge point was ±0.004inches on diameter, and that of the box was ±0.003 inches on diameter.

In the Example 1 configuration, there was at least some threadinterference in the vicinity of the seal area; consequently the sealingforce vs. distance characteristic resembled that of FIG. 9A.

EXAMPLE 2

The parameters, dimensions etc. were the same as for Example 1, exceptthat the box seal taper was 0.110 inches per inch on diameter, that thebox thread taper was selected to be 0.095 inches per inch, and the boxseal gauge point 42 was selected to be 0.500 inch from the box seat 46.

In the Example 2 configuration, there was no thread interference in thevicinity of the sealing area even when the coupling was made up. Andthere was slight mismatch between box and pin thread taper, the pintaper being slightly more inclined to the axis than the box taper, withthe result that the sealing force vs. distance characteristic resembledthat of FIG. 9D.

Terminology

The scope of the invention is as presented in the appended claims.

In the appended claims:

1. The term "connection" includes a coupling.

2. The phrase "relatively shallow" with reference to the slopes of thefrusto-conical sealing surfaces of the box and pin implies that:

(i) the taper is not so great as to give abearing-load-vs.-axial-distance characteristic similar to that of FIG.11A;

(ii) the taper is not so great as to create a significant risk of lossof seal due to thermal cycling of the coupling (i.e., alternatestressing of the coupling in tension and compression);

(iii) the taper is not so great as to reduce distal-end pin wallthickness unacceptably; and

(iv) the taper is nevertheless sufficient to avoid galling of thesealing surfaces during assembly of the coupling.

3. The term "slightly less" with reference to the slope of thefrusto-conical sealing surface of the pin relative to that of the boximplies that:

(i) the mismatch is sufficient to avoid abearing-load-vs.-axial-distance characteristic similar to that of FIG.10A;

(ii) the mismatch is not so great as to generate an effective contactingsealing area between the box and the pin that is unduly short in theaxial direction;

(iii) the mismatch is not so great as to give abearing-load-vs.-axial-distance characteristic similar to that of FIG.12A; and

(iv) the mismatch is not so great as to cause undue tilt of the pinduring assembly.

What is claimed is:
 1. A pipe connection of the type having a femalecomponent and a mating male component, each matingly threaded forconnection therebetween, and each provided with a sealing area adjacentthe threaded area, the sealing area of the coupling components beingformed as mating frusto-conical surfaces each having a preselected gaugepoint intermediate the axial ends of the sealing area, at least one ofsaid frusto-conical surfaces having controlled surface roughness; therespective sealing areas of the components being in axially alignedsealing engagement when the pipe connection has been assembled;characterized in that:
 2. A pipe connection as defined in claim 1,wherein the mismatch between the slopes of the frusto-conical sealingsurfaces of the male and female components in inches per inch is lessthan the nominal interference in inches at the gauge points of the saidsealing surfaces.(a) the slopes of the frusto-conical sealing surfacesare each relatively shallow; (b) the slope of the frusto-conical sealingsurface of the male component is slightly less than that of thefrusto-conical sealing surface of the female component; and (c) thesealing bearing load relative to axial distance of the contactingsealing surfaces of the assembled components being selected to lie abovea preselected design minimum but below force levels at which gallingoccurs, said bearing load of the assembled components qualitativelysimulating that of a pair of mating shrunk-fit circular cylindricalsealing surfaces.
 3. A pipe connection as claimed in claim 2 wherein themismatch in inches per inch does not exceed about 70% of the nominalinterference in inches.
 4. A pipe connection as defined in claim 2,wherein the slope of the frusto-conical sealing surface of the malecomponent is substantially the same as the slope of the thread pitchline of the male component.
 5. A pipe connection as defined in claim 2,wherein prior to assembly, to at least one of the sealing surfaces isapplied a high-temperature high-solids sealing lubricant containingmashable solid particles.
 6. A pipe connection as defined in claim 2,wherein the load flank faces of the load threads of both the male andfemale components are negatively inclined to the radial.
 7. A pipeconnection as defined in claim 6, wherein the angle of negativeinclination of the load flank faces does not exceed about 10 degrees,and the sum of the load flank angle and stab flank angle of the threadsto the radial is at least equal to about 15 degrees.
 8. A pipeconnection as defined in claim 2, wherein the female component isprovided with an interior torque shoulder forming an annular seatengageable by and mating with the distal end of the male component.
 9. Apipe connection as defined in claim 8, wherein the face of the annularseat is negatively inclined to the radial.
 10. A pipe connection asdefined in claim 1, wherein the load flank faces of the load threads ofboth the male and female components are negatively inclined to theradial.
 11. A pipe connection as defined in claim 10, wherein the angleof negative inclination of the load flank faces does not exceed about 10degrees, and the sum of the load flank angle and stab flank angle of thethreads to the radial is at least equal to about 15 degrees.
 12. A pipeconnection of the type having a female component and a mating malecomponent, each matingly threaded for connection therebetween, and eachprovided with a sealing area adjacent the threaded area, the sealingarea of the coupling components being formed as mating frusto-conicalsurfaces each having a preselected gauge point intermediate the axialends of the sealing area, at least one of said frusto-conical surfaceshaving controlled surface roughness; the respective sealing areas of thecomponents being in axially aligned sealing engagement when the pipeconnection has been assembled; characterized in that:(a) the elopes ofthe frusto-conical sealing surfaces are each relatively shallow; (b)when the connection is being assembled, interference between the sealingsurfaces occurs before any occurrence of any radially-directedinterference of the load threads of the male and female components inthe vicinity of the sealing surfaces; (c) upon complete assembly of theconnection, any interference between the load threads is insufficient totend to pry the sealing surfaces apart to an extent that would reducethe sealing bearing load below the minimum design value for theconnection; and (d) the sealing bearing load relative to axial distanceof the contacting sealing surfaces of the assembled components beingselected to lie above a preselected design minimum but below forcelevels at which galling occurs, said bearing load of the assembledcomponents qualitatively simulating that of a pair of mating shrunk-fitcircular cylindrical sealing surfaces.
 13. A pipe connection as definedin claim 12, wherein upon complete assembly of the connection, there isno thread interference creating a radial force between the saidfrusto-conical sealing surfaces.
 14. A pipe connection of the typehaving a female component and a mating male component, each matinglythreaded for connection therebetween, and each provided with a sealingarea adjacent the threaded area, the sealing area of the couplingcomponents being formed as mating frusto-conical surfaces each having apreselected gauge point intermediate the axial ends of the sealing area,at least one of said frusto-conical surfaces having controlled surfaceroughness; the respective sealing areas of the components being inaxially aligned sealing engagement when the pipe connection has beenassembled; characterized in that:(a) the slopes of the frusto-conicalsealing surfaces are each relatively shallow; (b) the slope of thefrusto-conical sealing surface of the male component is slightly lessthan that of the frusto-conical sealing surface of the female component;(c) the sealing bearing load relative to axial distance of thecontacting sealing surfaces of the assembled components being selectedto lie above a preselected design minimum but below force levels atwhich galling occurs, said bearing load qualitatively simulating that ofa pair of mating shrunk-fit circular cylindrical sealing surfaces; (d)when the connection is being assembled, interference between the sealingsurfaces occurs before any occurrence of any radially-directedinterference of the load threads of the male and female components inthe vicinity of the sealing surfaces; (e) upon complete assembly of theconnection, any interference between the load threads is insufficient totend to pry the sealing surfaces apart to an extent that would reducethe sealing bearing load below the minimum design value for theconnection.
 15. A pipe connection as defined in claim 14, wherein uponcomplete assembly of the connection, there is no thread interferencecreating a radial force between the said frusto-conical sealingsurfaces.
 16. A pipe connection as defined in claim 14, wherein themismatch in inches per inch between the slopes of the frusto-conicalsealing surfaces of the male and female components is less than thenominal interference in inches at the gauge points of the said sealingsurfaces.
 17. A pipe connection as claimed in claim 16 wherein themismatch in inches per inch does not exceed about 70% of the nominalinterference in inches.
 18. A pipe connection as defined in claim 16,wherein the load flank faces of the load threads of both the male andfemale components are negatively inclined to the radial.
 19. A pipeconnection as defined in claim 18, wherein the slope of thefrusto-conical sealing surface of the male component is substantiallythe same as the slope of the thread pitch line of the male component.20. A pipe connection as defined in claim 18, wherein the angle ofnegative inclination of the load flank faces does not exceed about 10degrees, and the sum of the load flank angle and stab flank angle of thethreads to the radial is at least equal to about 15 degrees.
 21. A pipeconnection as defined in claim 20, wherein the female component isprovided with an interior torque shoulder forming an annular seatengageable by and mating with the distal end of the male component. 22.A pipe connection as defined in claim 21, wherein the face of theannular seat is negatively inclined.
 23. A pipe connection as defined inclaim 22, wherein prior to assembly, to at least one of the sealingsurfaces is applied a high-temperature high-solids sealing lubricantcontaining graphite particles.
 24. For use in a pipe connection of thetype having a female component and a mating male component, eachmatingly threaded for connection therebetween, and each provided with asealing area adjacent the threaded area, the sealing area of thecoupling components being formed as mating frusto-conical surfaces eachhaving a preselected gauge point intermediate the axial ends of thesealing area, at least one of said frusto-conical surfaces havingcontrolled surface roughness; the respective sealing areas of thecomponents being in axially aligned sealing engagement when the pipeconnection has been assembled; a female component of the foregoing typecharacterized in that:(a) the slopes of the frusto-conical sealingsurface thereof is relatively shallow; (b) the slope of thefrusto-conical sealing surface thereof is slightly greater than that ofthe frusto-conical sealing surface of the mating male component; and (c)the sealing bearing load on the sealing surface thereof relative toaxial distance of the contacting sealing surfaces of the assembledfemale and male components being selected to lie above a preselecteddesign minimum but below force levels at which galling occurs, saidbearing load when the pipe connection has been assembled qualitativelysimulating that of a pair of mating shrunk-fit circular cylindricalsealing surfaces.
 25. A female component for a female component for apipe connection as defined in claim 24, wherein the mismatch between theslopes of the frusto-conical sealing surfaces of the male and femalecomponents is less than the nominal interference in inches at the gaugepoints of the said sealing surfaces when the connection has beenassembled.
 26. A pipe connection as defined in claim 24, wherein theload flank faces of the load threads of both the male and femalecomponents are negatively inclined to the radial.
 27. A pipe connectionas defined in claim 24, wherein the load flank faces of the load threadsof both the male and female components are slightly negatively inclinedto the radial, the angle of negative inclination of the load flank facesnot exceeding about 10 degrees.
 28. A female component for a pipeconnection as defined in claim 24, wherein the mismatch between male andfemale thread pitch line angle is less than that which would afford 50%contact between the distal threads of the male component and the matingthreads of the female component.
 29. For use in a pipe connection of thetype having a female component and a mating male component, eachmatingly threaded for connection therebetween, and each provided with asealing area adjacent the threaded area, the sealing area of thecoupling components being formed as mating frusto-conical surfaces, atleast one of said frusto-conical surfaces having controlled surfaceroughness; the respective sealing areas of the components being inaxially aligned sealing engagement when the pipe connection has beenassembled; a female component of the foregoing type characterized inthat:(a) the slopes of the frusto-conical sealing surface thereof isrelatively shallow; (b) when the connection is being assembled,interference between the sealing surfaces occurs before any occurrenceof any radially-directed interference of the load threads of the maleand female components in the vicinity of the sealing surfaces; (c) uponcomplete assembly of the connection, any interference between the loadthreads is insufficient to tend to pry the sealing surfaces apart to anextent that would reduce the sealing bearing load below the minimumdesign value for the connection; and (d) the sealing bearing loadrelative to axial distance of the contacting sealing surfaces of theassembled components being selected to lie above preselected designminimum but below force levels at which galling occurs, said bearingload of the assembled components qualitatively simulating that of a pairof mating shrunk-fit circular cylindrical sealing surfaces.
 30. For usein a pipe connection of the type having a female component and a matingmale component, each matingly threaded for connection therebetween, andeach provided with a sealing area adjacent the threaded area, thesealing area of the coupling components being formed as matingfrusto-conical surfaces, at least one of said frusto-conical surfaceshaving controlled surface roughness: the respective sealing areas of thecomponents being in axially aligned sealing engagement when the pipeconnection has been assembled; a female component of the foregoing typecharacterized in that:(a) the slopes of the frusto-conical sealingsurface thereof is relatively shallow; (b) the slope of thefrusto-conical sealing surface thereof is slightly greater than that ofthe frusto-conical sealing surface of the mating male component; (c)when the connection is being assembled, interference between the sealingsurfaces occurs before any occurrence of any radially-directedinterference of the load threads of the male and female components inthe vicinity of the sealing surfaces: (d) upon complete assembly of theconnection, any interference between the load threads of the femalecomponent and the mating male component is insufficient to tend to prythe sealing surfaces apart to an extent that would reduce the sealingbearing load below the minimum design value for the connection and (e)the sealing bearing load relative to axial distance of the contactingsealing surfaces of the assembled components being selected to lie abovea preselected design minimum but below force levels at which gallingoccurs, said bearing load of the assembled components qualitativelysimulating that of a pair of mating shrunk-fit circular cylindricalsealing surfaces.